Consideration of dynamic stall in rotorcraft airfoil design

Traditional rotorcraft airfoil design is based on steady-flow aerodynamics that are not representative of helicopters in forward flight. This is particularly true at high speed and high thrust conditions, when the rotor is susceptible to dynamic stall and its many negative consequences. The high loads associated with this phenomenon often become major drivers in the structural and aerodynamic design of a rotor. In this paper, traditional design requirements are revisited to include the effects of dynamic stall and new ways to qualifying rotorcraft airfoils are proposed. Design studies that demonstrate the role of trailing-edge separation in the dynamic stall behavior of airfoils are presented. The design manipulations are handled by an inverse-design, conformai mapping method, and unsteady Reynolds-averaged Navier Stokes equations (URANS) are used to predict the performance of airfoils under pitch motion. It is found that a proper tailoring of the trailing-edge separation development can provide adequate dynamic stall characteristics and minimize penalties in drag and nose-down pitching moment. The applied methodology can aid with the design of airfoils that are more suited for application at high loading conditions.